Separation of oxygen from heated, elevated pressure air streams produced by gas turbines can readily be accomplished by oxygen-selective, ion conducting ceramic membranes because gas turbines produce more high temperature air than is required to support combustion within the turbine. In fact, there is a sufficient excess of high temperature air to allow for significant quantities of oxygen to be extracted as a by-product.
There are a number of references in the prior art that disclose integrations of gas turbines with oxygen separators that employ oxygen-selective, ion conducing ceramic membranes (hereinafter referred to in the specification and claims as "oxygen-selective ceramic membranes"). For instance, J. D. Wright (et al., "Advanced Oxygen Separation Membranes", pp 33-61 (1990) discloses an integration in which compressed air is indirectly heated to the requisite membrane operating temperature by a fired heater. The air is then passed through the retentate side of the separator where a portion of the contained oxygen is transferred to the permeate side by a pressure driven ion conducting ceramic membrane. The oxygen depleted retentate is heated in a fired heater to turbine inlet temperature and is then expanded in a turbine to produce power. The fired heater contains a heat exchange coil for heating the separator feed. A similar integration is shown in U.S. Pat. No. 5,516,359. In this patent, air is compressed to an elevated pressure and is heated to a membrane operating temperature by a burner or by indirect heat exchange. The heated compressed air is then introduced to the retentate side of a membrane separator that extracts oxygen from the air. The oxygen depleted retentate is further heated to a turbine inlet temperature by direct combustion before being expanded in a turbine to generate power. U.S. Pat. No. 5,562,754 discloses the introduction of steam into the oxygen depleted retentate stream as a replacement for the separated oxygen and also deploys steam as a sweep gas for the permeate side of the membrane to improve the driving force for oxygen transfer.
U.S. Pat. No. 5,852,925 describes different process options that are especially suited for retrofitting existing installations. In one option, only a portion of the compressed air stream is processed by the membrane separator. The resultant oxygen depleted retentate is combined with a stream that has bypassed the separator prior to turbine expansion. Another option provides a separate air compressor to supply the membrane separator. The oxygen depleted retentate is heated in a second stage combustor and is then expanded in a turbine.
U.S. Pat. No. 5,865,878 introduces various concepts of integrating an oxygen-selective ceramic membrane with a gas turbine in which such reactants as steam and natural gas are introduced into the permeate side of the membrane separator to react with the permeated oxygen to form desired products such as syngas.
U.S. Pat. No. 5,820,654 discloses a process and apparatus in which oxygen is extracted from a heated oxygen containing stream by an oxygen-selective ceramic membrane in which the oxygen product is cooled through indirect heat transfer with a portion of the incoming air stream. The gas separation and cooling are integrated within a single apparatus to maximize the use of conventional materials of construction.
All of the foregoing references disclose separator-gas turbine integrations that require the use of ancillary equipment such as heat exchangers and long piping systems for extracting air and re-injecting oxygen depleted air. As may be appreciated, such equipment and piping adds to the complexity and expense of the integration of membrane separator and gas turbine. Additionally, long piping runs produce pressure drops and difficulties in providing the separator with a uniform flow distribution.
As will be discussed, the present invention provides oxygen separators and methods, employing oxygen-selective ceramic membranes, that are designed for integration with a gas turbine without the use of long piping runs. As a result, the pressure drop involved in handling the large air flow between the components of the system is minimized and flow distribution problems are reduced.